Concrete structures reinforced with iron or fiber rods or other reinforcing materials (hereinafter, “concrete structures”) are used in an extremely wide range of applications, especially in the fields of civil engineering and construction, because of their excellent mechanical strength and durability and low cost. They are used, for example, for bridge footing for highways and railways; tunnels; pillars, beams, and walls for architectural structures; and concrete precast products such as Hume concrete pipes and manholes.
In these concrete structures, steel materials such as iron rods corrode if oxygen, water, or chloride ion seeps inside. Corrosion products, formed by corrosion of such a steel material, expand in volume and cause cracks in the concrete structure, which accelerates the corrosion of the steel material. The steel material could decrease in cross-section, degrading the strength and other physical properties of the concrete structure. Thus, various methods have been developed to prevent the corrosion of steel materials for minimum maintenance in view of extended lifetime and/or life cycle cost of the concrete structure. Electrolytic protection is a method that prevents corrosion of steel materials in the concrete structure.
Electrolytic protection can be classified into impressed current systems and galvanic anode systems. An impressed current system operates on a DC power source to pass a protection current between an auxiliary electrode as an anode and a target object as a cathode (since the protection current is a direct current, the method requires a DC power source). Meanwhile, a galvanic anode system uses an electrochemically active metal (target object, generally a steel material in a concrete structure) as a cathode and an electrochemically active sacrificial metal (zinc, aluminum, or alloy of these metals) having a higher dissolution potential than the cathode as an anode in order to generate a potential difference between the cathode and the anode so that a protection current is provided.
In these methods for electrolytic protection, a sheet- or panel-shaped anode may be disposed on the surface of the concrete structure. When this is the case, an electrically conductive water retention material (backfill) is provided between the surface of the concrete structure and the anode to fill a gap at the interface to lower electrical resistance.
The electrically conductive water retention material (backfill) is preferably a gel so that it can follow rough surface of a porous material such as the concrete structure. Examples of such a water retention material (backfill) in gel form include the adhesive hydrogel of Patent Document 1 to which an electrolyte is added, the electrically conductive, non-electrolyte-based polymer gel of Patent Document 2, the backfill of Patent Documents 3 and 4 for use in electrolytic protection, and the ionically conductive medium of Patent Document 5.
These exemplary electrically conductive water retention materials in gel form are used in electrolytic protection and may also be used, for example, as a material for a non-polarized electrode installed on the surface of the ground or bedrock in an electric geological survey (see, for example, Patent Document 2) and as an electrically conductive material, for example, in detection of damage of a waterproof sheet in a waste disposal site (see, for example, Patent Documents 6 and 7).
The adhesive hydrogel of Patent Document 1 is a hydrogel containing a polymer matrix of a copolymer of a nonionic polymerizable monomer and a crosslinking monomer, water, a humectant, and a copolymer of acrylic acid and methacrylic acid. The adhesive hydrogel, due to its adhesiveness, is suited for use as a general pressure sensitive adhesive material even when no electrical conductivity is imparted to it.